[Rob Weiskirch]

This is on my dad's boat. We just put a 8-71 and water to air intercooler on a 468 chevy. We are using the Demon 750 blower carbs. The cam is a very mild hydraulic blower grind with around 234/238 at .050 on a 114.

Before start up we made sure transfer slot was a perfect square on both carbs. Engine idles with a "surge" between 1000 and 1500 rpm. This is with mixture screws 1.5 turns out. Surge goes away when leaning mixture screws to .5 turns out. At this point motor idles at 1300 with 12" of vacuum. This is with 16 initial timing with total of 30 all in by 3000. This is as slow as I can get it to idle. From what I have read this "surge" is pretty common on blown applications.

Now the tricky part. At the lake and put boat in gear motor will basically die. Keep in mind when putting a boat in gear the engine is very loaded down.....think of putting your race car in gear with a stock stall converter!

The carbs do not have holes drilled in the throttle plates. I do not think this is a issue as I have not had to increase the idle speed screw any from initial start up (remember the transfer slot is correctly adjusted).

I did increase the diameter of the idle air bleeds to .072 from .070 with no change where the idle mixture screws wanted to be for best vacuum reading.

Can anyone help?

Thanks, Rob

[Rob Weiskirch]

This is on my dad's boat. We just put a 8-71 and water to air intercooler on a 468 chevy. We are using the Demon 750 blower carbs. The cam is a very mild hydraulic blower grind with around 234/238 at .050 on a 114.

Before start up we made sure transfer slot was a perfect square on both carbs. Engine idles with a "surge" between 1000 and 1500 rpm. This is with mixture screws 1.5 turns out. Surge goes away when leaning mixture screws to .5 turns out. At this point motor idles at 1300 with 12" of vacuum. This is with 16 initial timing with total of 30 all in by 3000. This is as slow as I can get it to idle. From what I have read this "surge" is pretty common on blown applications.

Now the tricky part. At the lake and put boat in gear motor will basically die. Keep in mind when putting a boat in gear the engine is very loaded down.....think of putting your race car in gear with a stock stall converter!

The carbs do not have holes drilled in the throttle plates. I do not think this is a issue as I have not had to increase the idle speed screw any from initial start up (remember the transfer slot is correctly adjusted).

I did increase the diameter of the idle air bleeds to .072 from .070 with no change where the idle mixture screws wanted to be for best vacuum reading.

Can anyone help?

Thanks, Rob

[johnta1]

Why not set the idle with the boat in gear?
Shouldn't matter what it idles at while dis-engaged.

Or just throttle up a little before putting it in gear.

With it in gear I would think it would be under boost conditions,so it wouldn't surge as much.

[Tom Vaught]

Do your demon carbs have the power valves referenced to the intake manifold? (if they
are truly blower carbs there should be a nipple
at the base of each carb to allow this.

What are the power valves set at?

Where are you taking the manifold vacuum reading? Under the
carbs or in the intake?

Tom V.

[Rob Weiskirch]

Tom,
Yes, they are boost referenced primary with #65 power valves. The secondary just has plugs. The boost reference and my vacuum reading is coming from the intake manifold BELOW the blower.

I just realized today that these particular Demon carbs have what is called "idle eaze" base plate. There is a adjustment screw down the middle of the main body into the throttle body (where you screw the air cleaner stud in). Turning this adjustment screw I believe provides for more idle air.

Before the "idle eaze" screw was completely shut. Today I opened the screw 1.5 times and opened the idle air bleeds to .079. This helped the idle a bunch.....1100 rpm with 12 inches of vacuum and no hunt or surge.

True test will be in the water with a load in gear as today I was just on the trailer in nuetral.

[KEN CROCIE]

I consult with a local marine rigging co.and my son Kevin is the full time engine buider. With the 8-71 on a BBC , we lock out the ign advance for a better and less "rolling" idle .I would use a 10.5 power valve to get that circuit started sooner.shoot for a lower idle speed .with a locked out dist.you should be able to idle @ 900 rpm-in fact that's where we set the idle usually with that size cam and an outdrive. johnta1--It's recommended that the engine idle less than 1000 rpm because the outdrives don't like being shifted in and out of gear above that rpm---makes expensive noises.Rob: If you need any more info I would need more particulars on the boat itself.